While I was being silly crashing helicopters into the ocean, I noticed that the larger ones (Mi-48, GhostHawk, Ah99, Mohawk etc.) "bounce" on impact with the water at certain angles. If you are impacting the water at a 45 degree angle or so, the helicopter, on impact, will have its speed reduced and will be thrust upward with the helicopters nose facing upward. Hummingbird and Pawnee helicopters seem to just crash into the water as they should, unlike the larger helis.